It is known that color images may be obtained from imagewise exposed silver halide photographic elements by development with a primary aromatic amine color developing agent in the presence of a color coupler. The oxidized color developing agent formed in the areas of silver halide development couples with the coupler to form a dye. The coupler is normally incorporated in the sensitive photographic element.
It is also known that 5-pyrazolones in which the 4-position of the pyrazolone ring is free, that is having only hydrogen substituents (4-equivalent magenta couplers), can be used as magenta couplers in color photographic elements to provide magenta dye images having useful properties. Examples of such couplers are the 4-equivalents 3-anilino-5-pyrazolone couplers described in, for example, U.S. Pat. Nos. 3,519,429, 3,907,571, 3,928,044, 3,935,015 and 4,199,361. However, 4-equivalent 5-pyrazolone couplers have a number of disadvantages, as they require four equivalents of silver to produce each molecule of dye, are sensitive to certain chemical vapors, for example formaldehyde, and have poor dye light and dye dark stability. These drawbacks can be overcome by using so-called 2-equivalent 5-pyrazolone magenta couplers in which a substituent is introduced into the coupling position (4-position) of the coupler and eliminated as a leaving group (coupling-off group or splitting-off groups) during the color development process, thus requiring only two equivalent of silver in order to produce each molecule of dye.
Among coupling-off groups known in this connection are the arylthio groups described, for example, in U.S. Pat. Nos. 3,227,554, 3,701,783, 3,935,015, 4,351,897, 4,413,054, 4,556,630, 4,584,266, 4,740,438, 4,853,319, 4,876,182, 4,900,657, 4,929,540, 4,942,116, 5,250,407, 5,262,292, and 5,256,528; WO 88/04795, 92/18902, and 93/02393; EP 341,204, and GB 1,494,777.
Methods for preparing 2-equivalent 4-arylthio-5-pyrazolone magenta couplers are described in, for example, U.S. Pat. Nos. 3,227,554, 3,701,783 and Research Disclosure 13806 (1975). The method disclosed in U.S. Pat. Nos. 3,227,554 and 3,701,783 comprises converting an arylmercaptan into the corresponding arylsulfenyl halide and reacting the arylsulfenyl halide with a 4-equivalent 5-pyrazolone magenta coupler to introduce the arylthio group into the active coupling position of the coupler. Research Disclosure 13808 describes a method which comprises adding bromine to a solution of the 4-equivalent 5-pyrazolone coupler and arylmercaptan. The methods disclosed in U.S. Pat. Nos. 3,277,554 and 3,701,783 and in Research Disclosure 1308 have been further modified by using a diaryldisulfide instead of arylmercaptan, chlorine instead of bromine or presence of a base, as disclosed in, for example, U.S. Pat. Nos. 4,351,897, 4,853,319, 4,900,657, 4,929,540, 4,942,116, and 4,876,182, EP 341,204, GB 1,494,777, and WO 92/18902. In spite of the large number of methods disclosed in the prior art, these methods of synthesizing 2-equivalent 4-arylthio-5-pyrazolone magenta couplers have disadvantages as later will be shown by Comparative Examples, and there is still the need for more convenient, efficient and expeditious methods.
U.S. Pat. No. 4,032,346 discloses a method for preparing 2-equivalent couplers comprising at the active coupling position a group of formula --S--C(.dbd.Z)--B wherein Z represents an oxygen or sulfur atom and B represents a substituent group. The method comprises reacting a 4-equivalent coupler and a disulfide of formula B--C(.dbd.Z)--S--S--C(.dbd.Z)--B in the presence of an alkali catalyst and a solvent.
U.S. Pat. No. 4,293,691 discloses a method for preparing 2-equivalent couplers comprising at the active coupling position an alkylthio group. The method comprises reacting a 4-equivalent coupler A--H, wherein A is a yellow, magenta or cyan coupler residue, with an asymmetrical disulfide of formula ##STR1## wherein R is an alkyl, a cyclic alkyl or alkenyl group, or an aralkyl group, and R.sub.1 and R.sub.2 are substituent groups, in the presence of a base and a solvent to obtain a 2-equivalent coupler of formula A--S--R.
The methods disclosed in U.S. Pat. Nos. 4,032,346 and 4,293,691 are not useful for the synthesis of 2-equivalent 4-arylthio-5-pyrazolones, as will later be shown by Comparative Examples.
The present invention provides an improved process for preparation of 2-equivalent 4-arylthio-5-pyrazolone magenta couplers which overcome drawbacks of prior art processes and provides a simple reaction mechanism.